Dutch EU blue card exchange in Germany

Hello, I have been living in the Netherlands for the last 5 years, at the beginning I had a Dutch residence permit but from 4th of April 2016 I had EU Blue card. 18 months it was related to the Netherlands but they have already finished. Today I started working in the German company in Berlin and applied to the German Blue card. I expected that my Dutch years with EU Blue card will be counted and I will be able to apply to the permanent permit in 6-8 months. But the guy in the LABO Landesamt für Bürger- und Ordnungsangelegenheiten told me they are not going to take into account these 20 months in the Netherlands and I will be able to apply to the permanent permit only after 22 months in Germany with B1 language knowledge. Could you explain me why my Dutch period wasn't taken into account?